Electro-optic devices using liquid crystals which have been developed and put into practical use to date include those using nematic liquid crystals, such as a DSM mode, a TN mode, a G-H mode, and an STN mode. However, such devices using nematic liquid crystals have a very slow electro-optic response and require a switching time from several to several ten milliseconds and are thus limited in their range of application. The slow response of these elements using nematic liquid crystals is due to the fact that the torque of moving molecules, which is basically based on the anisotropy of their dielectric constant, is not very high.
In light of the above, Meyer et al developed ferroelectric liquid crystals which undergo spontaneous polarization (Ps) which have a strong torque, the torque being based on Ps x E (the applied electric field), and which thus have a high speed response on the order of microseconds, as disclosed in Le Journal de Physique, Vol. 36, L-69 (1975). Further, JP-A No. 63-307837 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses new ferroelectric liquid crystals, but has no disclosure on the "three states" concept hereinafter discussed.
Several high speed electro-optic devices using ferroelectric liquid crystals have been proposed to date. Typically, such devices include an element in which a twisted structure is untwisted by the force of wall surfaces, and two molecular alignment layers in parallel to the wall surface are varied by changing the polarity of an applied electric field as described, e.g., in JP-A No. 56-107216.
The use of a compound showing ideal two states having an electric field response waveform as shown in FIG. 1 is a prerequisite in the above described devices. However, such a compound exhibiting ideal two (bistable) states is not yet available. The so far synthesized bistable liquid crystals have a response waveform as shown in FIG. 2, not as shown in FIG. 1. When the state-of-the-art liquid crystals having a response waveform as shown in FIG. 2 are used, for example, in light switching circuits, since transmission gradually changes as the applied voltage changes from negative to positive, the desired results cannot be sufficiently achieved simply by changing the applied voltage between "on" and "off". Moreover, currently available bistable liquid crystals have difficulty in reaching a mono-domain state in their Sc.sup.* phase without an applied voltage, i.e., in reaching an ideal molecular orientation state, and easily undergo the defect or a molecular orientation disturbance called twist. Thus, it has been difficult to achieve the above ideal two states of molecular orientation over a wide range.
Further, because the threshold value (voltage at which luminance changes by a prescribed value) is low, dynamic driving is liable to suffer from a reduction in contrast or a reduction in the viewing angle.
Further, these conventional bistable liquid crystals do not exhibit a hysteresis loop as shown in FIG. 1 but exhibit hysterisis as shown in FIG. 2 so they have no memory effect. Therefore, it is necessary to continue applying a voltage .nu..sub.3 as shown in FIG. 2 or continue applying a high frequency for the liquid crystal to maintain a stable response in the Sc.sup.* phase, which, in either case, results in a considerable energy loss.
Thus, conventional electro-optic devices have many defects which need to be overcome, notwithstanding the strong demand for devices which make effective use of the characteristics of electro-optic devices to use an applied electric field to achieve molecular orientation of ferroelectric liquid crystals.